The present invention relates to a switching power supply and more particularly to a switchmode converter circuit having a parallel resonant circuit topology and a capability to operate in both buck and boost modes.
A converter circuit is used for producing one or more regulated DC output voltages of desired values. The converter generates the regulated output DC voltages from a DC line voltage (DC-DC conversion) or from and AC line voltage (AC-DC). In the latter case an input rectifier is provided for rectifying the input AC line voltage to the unregulated DC voltage that is used by the converter.
By way of background and introduction, switchmode converters operate by switching a DC input line voltage between two primary windings of a transformer which are wound out of phase with one another. The resulting output at the secondary winding is an AC pulse width modulated rectangular wave signal having an output magnitude which is related to the primary voltage by the turns ratio between the secondary and primary windings. This AC signal, at the secondary winding, is rectified, for example by passing it through a full wave rectifier bridge and capacitor filter combination, to obtain an essentially ripple-free output DC voltage. Additional secondary windings are provided if more than one DC outputs are needed.
The magnitude of the output DC voltage with respect to the primary voltage is established primarily by the turns ratio between the secondary and primary windings. It is however also affected by the voltage or current that is impressed on or through the primary windings.
To assure that the output voltage or voltages remain within their prescribed limits, at least one of the outputs is constantly monitored and conditions at the primary are controlled to assure that the output remains within its set limits.
The typical specification of a power supply includes the definition of the following items:
The nominal output voltages; PA1 The minimum and maximum load currents; PA1 The load regulation, e.g. the change in the output voltages for a specified change in any or all of the load currents; PA1 The line regulation, e.g. the allowed change in the output voltages for a specified change in the input voltage; PA1 The cross regulation, e.g. the allowed change in one specific output voltage when the current in other than this specified output is changed by a specified amount; PA1 The input line compliance, e.g. the ratio of the maximum to the minimum input line voltage over which the converter is capable of providing the specified performance. PA1 "Switchmode" technique refers to a converter which is based on "chopping" the DC input voltage at a high frequency (typically higher than 50 khz) to convert it to a pulse width modulated AC rectangular waveform having the desired average value. This technique must include a rectifying block to convert the AC to unfiltered DC, and must include an averaging output filter (usually an LC filter) to reduce the chopping frequency related ripple voltage levels to within the desired values. PA1 "Sampling frequency" refers to the frequency at which the controller samples and adjusts the output voltages. PA1 "Control bandwidth frequency" refers to the bandwidth of the feedback control loop. This control bandwidth determines the upper frequency limit for which the control circuit can reduce the influence of disturbing signals. PA1 "Voltage mode" refers to a switchmode converter wherein the pulse width time is controlled, and the current driving the primary of the transformer is influenced by other parameters such as input voltage and output filter inductance value. PA1 "Current mode" refers to a switchmode converter wherein the instantaneous current in the filter inductor is controlled independent of the other variables which influence this current in the voltage mode topology. PA1 "Continuous/discontinuous current switching" is used to denote a converter in which, respectively, the current flowing through the primary winding is continuous or discontinuous. In a discontinuous switcher the current through the primary drops to zero before the end of each control period. PA1 "Zero current turn off" is related to a "discontinuous current switching converter" and denotes the fact that the current through the switching element has subsided before certain switching elements in the converter are turned on or off. PA1 "Series resonant converter" refers to converters which employ a series resonant circuit topology in their DC-AC stage for processing the power from which the output DC voltages are generated. PA1 "Parallel resonance converter" refers to converters in which a parallel resonant circuit is used. PA1 "Q multiplication" is meaningful in relation to converters using resonant circuits and refers to the inherent ability of a resonant converter to develop a voltage which is several i.e. "Q" times larger than an input voltage to which the resonant circuit is connected. PA1 "Buck mode" refers to a converter in which the operating voltage at the primary winding of the converter is lower than the input voltage supplied to the converter. PA1 "Boost mode" is similar to buck mode except that the voltage at the primary is higher than the input voltage.
In one switchmode power supply described in U.S. Pat. No. 4,475,149, in the name of George C. Gallios and assigned to the assignee of the present application, (the contents of which are incorporated herein by reference) the AC line voltage is rectified by a bridge rectifier circuit to produce an unregulated input DC voltage. The unregulated DC voltage has an average or DC value which is subject to AC line voltage fluctuations. A DC input filter receives and filters the input DC voltage to reduce or eliminate ripple voltages therefrom. Thereafter a converter circuit is provided for producing a DC output voltage from the input DC voltage.
The converter "chops" the input DC voltage to produce AC voltage signals at secondary windings of an output transformer. The AC voltage signals at the secondaries are converted to the regulated output DC voltages in the output stage. An output stage includes rectifier and output filtering elements for the output DC voltages. A controller circuit in the DC-AC stage samples and monitors the output DC voltages and controls the voltage or current flowing through the primary windings of the output transformer to assure that the output DC voltages remain within certain set limits.
A typical converter arrangement is shown in FIG. 1 of the drawings.
Certain terms and concepts related to switchmode power supplies are defined and discussed below.
Besides the obvious need to maintain the output DC voltages within their prescribed limits in the face of input line or output load fluctuations, the converter designer must strive to design the converter to operate at a high efficiency. Efficient converters minimize heat generation. They are smaller in size, weigh less and need not include heat removal means such as heat sinks, fans, and the like. Inefficient converter operation results in part from a converter topology in which switching elements dissipate power each switching cycle.
It is desirable to utilize a switching speed which is as high as possible to allow electrical components to be made smaller at higher frequencies. Increasing switching speeds, however produces more switching cycles per unit time with a consequent reduction in efficiency.
The present invention is directed to a novel and unique switchmode converter which employs a parallel resonant circuit topology and operates at a fixed frequency. In addition, the invention features a unique input coupled inductor that will be described in detail later herein. The input coupled inductor is adapted to divert energy stored in this inductor at the end of a switching cycle back to the input DC voltage source and to prevent dissipation of that inductive energy in the switching elements of the converter. Regulation of the output DC voltages is effected by varying the phase of a control signal with respect to a voltage waveform that is generated by the converter's resonant components. The control signal adjusts output by pulse width modulation of the switching elements in the converter.
It has been customary to use a series resonant topology in which the switching elements of the converter are connected in series with the primary winding of the output transformer and further in series with a resonating inductor and a resonating capacitor.
Series resonant converters do provide certain advantages. They minimize turn on switching transient currents in the switching elements and in the output rectifiers. Another advantage relates to cases where such series resonant converters are operated in a discontinuous mode. In the latter case the losses in the switching elements are minimized and the series topology enables the use of SCR's for the switching elements.
Ideally, all the high frequency current flows in the series resonant converter topology exclusively through the switching elements, the primary winding, the resonating inductor and through the resonating capacitor. In actuality, however, the stray capacitance across the secondary of the output transformer is reflected to the primary of the output transformer. The stray capacitance, therefore, appears in parallel with the primary winding of the output transformer. As such the reflected array capacitance presents a serious problem for series resonant converters since a bypass shunt path is created for some of the resonant circuit current. Particularly in high voltage applications, where the turns ratio of the output transformer is high, very careful and extreme measures must be taken to minimize the stray capacitance. Otherwise, it can interfere with the operation of the converter and reduce its efficiency significantly.
Another disadvantage of series resonant converters arises when such converters operate in the "discontinuous mode". A given fixed amount of energy is transferred each switching cycle from the input line to the output. Control over and regulation of the output voltage is achieved by varying the frequency (control frequency) of operation. As a result, a large load current range requires a correspondingly large frequency shift range for control. Such a system is inherently inferior to and more complicated than a fixed frequency system.
A series resonant converter can be operated in a continuous current mode wherein the next switching cycle is turned on before current from a previous cycle has ceased. But the continuous mode loses the benefit of operating the switching elements when the current is equal to zero, and efficiency suffers. Furthermore, although the continuous mode of operation provides the advantage of higher power density, i.e. more power is generated from a given converter size, the allowable range of the controlling frequency is smaller. Therefore, the dynamic control range of the output power is accordingly limited.